Although the principles of the present invention may have utility in many areas, for convenience it will be described mainly in connection with liquid treatment of eyes. Typically the medical preparation has to be delivered in a fairly well defined volume to assure a specified dose to be delivered or absorbed. A large surplus cannot be allowed due to improper systemic physiological effects from absorbency in non-target tissues or drainage of excess amounts through the tear channel into the throat cavity or the inconveniences caused by overflow on face and clothes. Also price considerations apply for expensive medications. As an example, the treatment of glaucoma requires frequent daily administrations of e.g. prostaglandins, beta-blockers or other expensive active ingredients, all having other then the desired pressure relieving action when absorbed by other body tissues than the eye. Small volume dosing is negatively affected by even small uncontrolled or dead spaces in delivery equipments used. Moreover, medical preparation components may be sensitive to degradation or absorption at prolonged exposure to materials and extended surfaces present in delivery devices. Similar considerations apply for sterility preservation. With regard to stream quality, proper administration of small amounts is complicated by the fact that the active ingredients cannot enter the eye but through the limited area of the cornea. It is also necessary that the entire dose can be delivered before the triggered blink reflex closes the eyelid.
A large number of devices are already known for applying a determined quantity of a liquid medicinal product onto a part of the body, such as an ophthalmic solution on the surface of the eye. These devices generally rely on the principle of a syringe which can be either pre-filled with a determined quantity of liquid, or graduated to suck up said quantity of liquid contained in a separate bottle, or connected to a fixed receptacle in permanent communication with the dosing chamber of the syringe, as is described for example in one of the embodiments of U.S. Pat. No. 4,623,337. It will be observed that permanently feeding the dosing chamber from the receptacle via gravity means that neither the precision of the quantity of liquid to be ejected, nor the sterility thereof can be guaranteed. In these devices, the pressure exerted on the plunger, manually or automatically, is generally exerted in the same direction as that of the liquid jet, as is described for example in International Patent Application No. WO 92/20455.
The direction of the jet can sometimes be deviated by bent conduits, but it is then difficult to control the force with which the jet reaches its target. A device of this kind is like, for example, that disclosed in French Patent No. FR 2 647 757 for food products or cosmetics in liquid or paste-like form, for which respecting a given ejection pressure is of no importance.
In the case of a an ophthalmic solution, it is, however, very important not only to control very precisely the dose to be ejected for obvious reasons of safety and efficacy of the treatment, but also in order to be able to control the impact pressure of the liquid jet on the eye, which certain devices attempt to achieve by using an eyepiece or a spacing member applied to the periphery of the target to impose a fixed distance with respect to the liquid ejection orifice, as is disclosed for example in U.S. Pat. Nos. 4,623,337 and 5,836,911. It will be observed however that these devices do not always allow the impact force of the liquid jet to be reproduced when the pressure is exerted directly on the plunger manually.
Thus, the dispensing apparatuses of the prior art provide individual solutions to particular problems, but none of them allows all of the aforementioned problems to be simultaneously resolved.